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Astrophotography

A Runaway Star with a Story To Tell

Just five years ago the first hypervelocity star was confirmed, by Warren Brown of the Harvard-Smithsonian Center for Astrophysics. Now about 16 are known. These are stars moving through space so fast, upwards of several hundred kilometers per second, that they will escape the Milky Way's gravity and forever roam the intergalactic void. Their discovery had to wait for very efficient surveys; only about one in 100 million stars is going so fast.

Centered in this near-infrared Hubble image is the hypervelocity star HE 0437-5439, magnitude 16.4. It's not only going way too fast, it also seems to be a merged-binary "blue straggler."

NASA / ESA / O. Gnedin / W. Brown

Several possible mechanisms can fling a star with that much speed. But the most productive of them seems to be happening at a single, very special place. If a binary star falls close enough to the supermassive black hole at the Milky Way's center, the binary can be disrupted in such a way that one star is trapped around (or in) the hole and the other is slung away with preternatural velocity. This is thought to account for most of the escaping 16.

One of them, known as HE 0437–5439, is already 200,000 light-years out near the Large Magellanic Cloud, way beyond the usual definition of the Milky Way's limits. Brown and six colleagues have published a new study that presents a complex picture of its nature and history.

Other astronomers had argued that it probably came from the Large Magellanic Cloud (LMC) itself, rather than the Milky Way, based on the star's chemical makeup. Brown's group has laid that scenario to rest by measuring the star's proper motion on the sky for the first time. Two high-resolution Hubble images taken 3.5 years apart show that it's moving away not from the LMC, but from the Milky Way's center in Sagittarius.

The star is departing at 723 km per second as measured with respect to the Sun. Before it climbed out of the Milky Way's gravitational well, it must have been going 820 km/sec with respect to the galaxy. Even so, it must have taken 100 million years to get where it is now.

And that's a problem. Its spectrum shows it to be a young, main-sequence star of type B with about 9 solar masses. Such a massive main-sequence star can't be more than about 20 million years old.

Brown's group proposes an answer: the star was born near the Milky Way's center as a triple, consisting of a tight binary and a more distant third star. The third was the one captured by the black hole; the tight binary was flung off. As it raced out of the Milky Way, one or both of the binary's stars evolved to swell and engulf the other. The result was a "newborn" single star known as a blue straggler. Blue stragglers have long been familiar in globular clusters, where stellar encounters and mergers are fairly common, and elsewhere.

The team is now trying to determine the origins of four other hypervelocity stars on the fringes of the Milky Way. "We are targeting [other] massive B stars," says Brown. "These stars shouldn't live long enough to reach the distant outskirts of the Milky Way, so we shouldn't expect to see them there. The density of stars in the outer region is much less than in the core, so we have a better chance to find these unusual objects."

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